A keypad assembly is provided in an electronic device such as a mobile terminal to generate an electrical signal required for operation. The keypad assembly includes a lighting device to enable easy operation of an electronic device in a dark lighting environment, such as at night or in a dark location.
In general, an independent lighting-type keypad assembly that illuminates a key button by providing a light emitting element in each key button is well known. However, in an independent lighting-type keypad assembly, because the same number of light emitting elements as key buttons is required, manufacturing cost and power consumption are high.
In order to solve such a problem, a lighting type keypad assembly using a light conductive plate is used. This type of keypad assembly illuminates all key buttons with a small quantity of light emitting elements by widening a light emitting range using the light conductive plate. Because this type of keypad assembly is effective for cost reduction and power consumption reduction, the use of this type of keypad assembly has gradually increased. An example of the lighting type keypad assembly using the light conductive plate is described hereinafter.
FIG. 1 is a cross-sectional view illustrating a structure of a conventional keypad assembly, and FIG. 2 is a diagram illustrating operation of the conventional keypad assembly of FIG. 1.
Referring to FIGS. 1 and 2, a conventional keypad assembly 310 includes a keypad 320, electrically conductive dome 336, printed circuit board (PCB) 331, light emitting diode 341, and light conductive plate 343. The keypad 320 includes a plurality of key buttons 322. The keypad assembly 310 has a structure in which the key buttons 322 are arranged in an upper part of the light conductive plate 343 and in which two light emitting diodes (341, 342) are provided at one edge of the light conductive plate 343.
Light generated in each of the light emitting diodes (341, 342) is applied to a side surface of the light conductive plate 343 and diffused to the inside of the light conductive plate 343. As the light advances through the inside of the light conductive plate 343, reflective patterns 345 formed in a lower part of the light conductive plate 343 below the key buttons 322 reflect the light to key button light emitting areas 347 and transmit the light towards the key buttons 322. Accordingly, the key buttons 322 receive light to become illuminated.
However, in the conventional keypad assembly, much light loss is generated in the outermost sides of the light conductive plate. Due to the rectilinearity of light, much of the light applied through a side surface of the light conductive plate is discharged to the outside through other side surfaces of the light conductive plate, as indicated by arrows in FIGS. 1 and 2. Particularly, much light is lost through the side surface opposite to the side surface to which light is applied. Further, because the conventional keypad assembly has a light emitting angle of about 120 degrees, as displayed with a broken line in FIG. 2, light is not transmitted to two corners A and B of the side in which the light emitting diode is provided.
Accordingly, brightness of the key button deviates according to the key button position. In order to compensate the deviation of brightness and to secure light intensity, it is necessary to provide light emitting diodes at two or more positions. For this, as a plurality of light emitting elements is used, current consumption increases and cost increases.
According to an experimental result of the present applicant, it can be seen that 50% or more of the input light is not used to illuminate a key button and is lost. Current of about 20 mA is generally consumed per light emitting diode for a keypad backlight for illuminating the key buttons. In the above-described keypad assembly, it is confirmed that current of 40 mA or more is consumed for illumination of the keypad assembly.